I show them how I vacuumed.
One of the first things I show them is to vacuum in such a direction as to pull up the pile. Move your foot across the carpet N, S, E, W, see which way pulls up the pile/loops. I explain how the brush will always spin trying to pull the vacuum away from you so they can tell which way to stand. They will pull up more soil in half the time vacuuming in the right direction and since most people don't want to spend a lot of time vacuuming, may as well tell them how to do it effectively.
I also tell them that they need to vacuum slow on the forward pass as thats when most of the soil will come in, contrary to what most people think.
Also for them to use slow passes, esp when they use a bag vacuum, since they won't be able to tell when they are removing soil. They need to let the vacuum do its work and it can't if they are speed vacuuming. It might get the surface soil but could be leaving lots IN the carpet.
Check their filters, wash or replace. Check the manual to find all the filters. Some of these new vacuums have filters buried in plastic, need an engineering degree to find them!!
Oh yeah, if a bag vacuum, replace it by the time its half full, despite what the retailer will tell you, the bag has lost its effectiveness by this time.
Check the brush as it wears out over time.
If they want to check the air flow, don't put your hand over the hose but rather put your finger in the end and feel how much its pulling. If its not pulling at your finger much, may need to have it looked at.
Sometimes I've even recommended how to stand when they tell me they have back issues.
Just some basics.